In recent years, in the information recording field, studies on an optical information recording method have been pursued in various places. This optical information recording method has advantages of being able to record and reproduce in a non-contact way and to be compatible with respective memory formats of a read-only type, a recordable type and a rewritable type, and a wide use range from industrial use to consumer use is considered as a method of realizing a large capacity file at a low cost.
A large capacity of an optical recording medium (hereinafter, also referred to as an optical disk) for the various optical information recording methods as above has been attained by making a spot size on a focal surface small by making a wavelength of a laser light as a light source to be used in the optical information recording method short and adopting an objective lens having a large numerical aperture.
For example, a CD (compact disk) has a laser light wavelength of 780 nm, the numerical aperture (NA) of an objective lens of 0.45 and a recording capacity of 650 MB, while a DVD-ROM (digital versatile disk read-only memory) has a laser light wavelength of 650 nm, the NA of 0.6 and a recording capacity of 4.7 GB.
Furthermore, in an optical disk system of the next generation, a large capacity of 22 GB or more has been studied by attaining a laser light wavelength of 450 nm or less and the NA of 0.78 or more by using an optical disk wherein a thin light transmitting protective film (a cover layer) of, for example, 100 μm or so is formed on an optical recording layer.
As a method of obtaining a still higher density, a multilayer optical disk wherein two or more optical recording layers are stacked in the vertical direction via a physical interlayer has been developed. Below, an optical disk having a plurality of optical recording layers as such will be also called a multilayer optical disk and an optical disk having one optical recording layer will be also called a single layer optical disk.
In an optical disk system wherein a laser light wavelength is made short and the numerical aperture of an objective lens is made large for attaining a large capacity as above, there are disadvantages that spherical aberration is liable to occur and a focal depth becomes shallow, etc., and the light transmitting layer is demanded that the film thickness does not become out of a predetermined value and a film thickness irregularity is small.
In a multilayer disk made by stacking optical recording layers in the vertical direction, not only the light transmitting layer, but the interlayer is also demanded to have a predetermined film thickness and a small irregularity of the film thickness in the same way.
As a method of forming the interlayer provided between the optical recording layers, conventionally, a method of forming a predetermined film thickness by spin-coating using an ultraviolet curing resin to secure necessary properties for recording and reproducing has been general.
However, in the method of forming the interlayer by spin-coating using an ultraviolet curing resin as above, there was a disadvantage that irregularity of the film thickness is liable to occur on inner and outer circumferences of the optical disk and the improvement has been demanded.
Furthermore, there have been disadvantages of being susceptible to the influence of warps of a disk substrate and a transfer substrate and having a weakness for dusts and burrs.